This invention relates to novel aromatic copolyesters that are optically anisotropic in the melt and that are derived from (a) a mixture of aromatic diols, (b) aromatic dicarboxylic acids, and (c) t-butyl-4-hydroxybenzoic acid. The key to this invention is the incorporation of t-butyl-4-hydroxybenzoic acid into the claimed copolyesters. These polymers range from glassy to semi-crystalline and are unexpectedly melt processable at temperatures generally at or below 350 degrees C.
Optically anisotropic melt forming aromatic copolyesters based on various combinations of aromatic diols, aromatic dicarboxylic acids and aromatic hydroxycarboxylic acids are well known in the art. Due to the melt anisotropy of such aromatic copolyesters, shaped articles such as films and fibers made from them tend to have excellent mechanical properties. Unfortunately, the copolyesters also tend to have very high crystalline melting temperatures, a factor which makes it difficult to process them in conventional melt-processing equipment.
The art is replete with attempts to improve the melt processability of anisotropic aromatic copolyesters (generally, by lowering crystalline melting temperatures) by altering one or more of the components in the polymer. For example, some have tried to achieve lower melting points by incorporating different substituents on the aromatic rings or by using some aromatic rings having chain extending linkages which are neither parallel nor coaxial. Numerous attempts have involved altering the aromatic hydroxycarboxylic acid component of the copolyester, as is the case in the present invention.
U.S. Pat. No. 4,447,593 to Funakoshi et al. discloses an aromatic polyester having melt anisotropy and an optional hydroxycarboxylic acid component which can be a mono-substituted p-hydroxybenzoic acid. The substituted p-hydroxybenzoic acid is preferably substituted by an alkyl group having 1 to 4 carbon atoms or by a halogen atom. The compound 3-t-butyl-4-hydroxybenzoic acid is included as an example of a suitable substituted p-hydroxybenzoic acid (column 5, lines 56-57, and Example 24 describes a polymer derived from (diphenylmethyl)hydroquinone, terephthalic acid, and 3-t-butyl-4-hydroxybenzoic acid.
U.S. Pat. No. 4,603,190 to Dicke et al. discloses thermotropic aromatic polyesters derived from (a) optionally substituted p-hydroxybenzoic acid, (b) isophthalic and optionally terephthalic acid, (c) hydroquinone and (d) certain polyaromatic diols. The preferred p-hydroxybenzoic acids are p-hydroxybenzoic acids substitued in the nucleus by C.sub.1 -C.sub.4 alkyl or alkoxy, C.sub.6 -C.sub.10 aryl or C.sub.7 -C.sub.12 alkaryl groups or halogen. There is no express disclosure of t-butyl-4-hydroxybenzoic acid, and it is indicated that unsubstituted p-hydroxybenzoic acid is the particularly preferred hydroxycarboxylic acid. U.S. Pat. Nos. 4,600,764 and 4,564,669 to Dicke et al. disclose similar thermotropic aromatic polyesters incorporating an optionally substituted p-hydroxybenzoic acid as described in U.S. Pat. No. 4,603,190.
U.S. Pat. No. 4,536,561 to Schmidt et al. discloses thermotropic, wholly aromatic polyesters based on (a) p-hydroxybenzoic acid, (b) 3-chloro-4-hydroxybenzoic acid, (c) isophthalic acid, (d) hydroquinone and (e) certain polyaromatic diols. The p-hydroxybenzoic acid (a) may be substituted by C.sub.1 -C.sub.4 alkyl or alkoxy, C.sub.6 -C.sub.10 aryl or C.sub.7 -C.sub.12 alkaryl groups. As in the previously mentioned patents to Dicke et al., there is no express disclosure of t-butyl-4-hydroxybenzoic acid, and it is indicated that unsubstituted p-hydroxybenzoic acid is the particularly preferred hydroxycarboxylic acid.
U.S. Pat. No. 4,299,756 to Calundann discloses melt processable wholly aromatic polyesters derived from (I) 2- or 3-phenyl-4-hydroxybenzoic acid, (II) 4-hydroxybenzoic acid, (III) at least one dioxy aryl compound which can be either hydroquinone or certain polyaromatic diols, and (IV) at least one dicarboxyl aryl moiety such as terephthalic acid. The patent thus discloses the incorporation in a polyester of a 4-hydroxybenzoic acid substituted with a phenyl group; however, there is no mention of the use of a 4-hydroxybenzoic acid substituted with any other substituent, e.g., a t-butyl substituent.
Despite the wealth of work which has been done in developing new optically anisotropic melt forming aromatic copolyesters, there still exists a need for such polymers which can be easily and economically processed. It is therefore an object of this invention to provide optically anisotropic melt forming copolyesters which possess a high degree of processability and outstanding mechanical properties. Another object of this invention is to produce optically anisotropic melt forming aromatic copolyesters which may be processed by thermoplastic shaping to produce moldings, fibers, and films which have a high strength and stiffness.
Surprisingly, it has been found that the copolyesters described below have this desired combination of advantageous properties.